A sensor network is an important technical infrastructure for implementing ubiquitous computing technologies. The sensor network may be operated in wired or wireless connection. In the sensor network, since each node uses a battery having a limited energy capacity, it is most important to transmit sensing data by using minimum energy. In addition, in order to adapt the sensor network to a real-time application system, network latency in a large-scale network needs to be minimized. Therefore, in the sensor network requiring real-timing monitoring, a protocol capable of minimizing power consumption of the battery and having small network latency is needed.
Each of nodes constituting a wireless sensor network (WSN) performs operations such as sensing peripheral information, computing, and wireless communication by using limited battery energy. In the case where a typical MAC protocol such as existing MANET and IEEE 802.11 is adapted to the sensor network, problems such as packet collision/latency, overhearing, control packet overhead, and idle listening, which consume energy in the wireless network, occur. In particular, in the idle listening, since a sensor node always operates in an active state even in a duration where a communication function is not required, much energy consumption occurs. In order to complement the problems, a MAC (medium access control) protocol used in a sensor network reduces energy consumption by using a method of minimizing power consumption by operating in a sleep state at a normal time and periodically waking up and operating in an active state. As an example of a sensor MAC protocol using such a method, there are a Sensor-MAC, a Timeout-MAC, a B-MAC, and the like.
However, in the aforementioned sensor MAC protocols, energy efficiency is decreased. If a duty cycle is allowed to be decreased in order to increase the energy efficiency, the network latency occurs in proportion of the decrease of the duty cycle. In other words, the aforementioned sensor MAC protocols, the energy consumption and the network latency have an inversely proportional relationship therebetween. Therefore, the aforementioned sensor MAC protocols have problems in the case where a limit power source such as a battery is used and real-time application is required.
In order to solve the problems, Korean Patent No. 10-656385, titled “Communication Method in Real-time Wireless Sensor Network Having linear structure” discloses a communication protocol having a linear structure adapted to a wireless sensor network. Although the communication protocol disclosed in the Patent Document can configure an efficient wireless sensor network as a whole, the communication protocol has a problem in that downstream for transmitting data or commands from a sink node to a terminal node cannot be configured.
In order to solve the problem, the inventor of the present invention discloses a technology in Korean Patent Application 10-2007-0008935. FIGS. 1 and 2 are conceptual diagrams illustrating data transmission/reception structures of the wireless sensor networks disclosed in the aforementioned Patent Document. Specifically, FIG. 1 is a packet diagram illustrating example TDMA packets in a conventional sensor network of a linear structure. Referring to FIG. 1, an active period 100 and an inactive period 140 each having a predetermined time length are repeated in each node of the sensor network according to the present invention. The active period 100 includes a downstream duration 110, an intermission duration 120, and an upstream duration 130. Each of the downstream duration 110 and the upstream duration 130 sequentially includes a RX interval 112, 132, a TX interval 114, 134, and an ACK interval 116, 136, respectively. On the other hand, each data packet transmitted or received by each node preferably includes its own identification and link direction information. The link direction information indicates a propagation direction of a corresponding packet, e.g., any one of the downstream duration and the upstream duration. If the sensor network includes n nodes (i.e., the first node, the second node, . . . , the nth node), the first to nth nodes are linearly connected, the first node is also called a sink node, and the bottommost nth node is also called a terminal node. During the downstream duration 110 of the active period, the sink node transmits commands to the terminal node. During the upstream duration 130, the terminal node transmits data to the sink node in response to the command from the sink node. As shown in FIG. 1, the sensor network according to the present invention has both of the downstream duration for transmitting commands and the upstream duration for transmitting data from each node in a single period. Therefore, the sink node can transmit commands as well as receive processing results for the commands within a single period. FIG. 2 is a packet diagram illustrating other example TDMA packets in the conventional sensor network with the linear structure. Referring to FIG. 2, an active period 400 and an inactive period 440 each having a predetermined time length are repeated in each node of the conventional sensor network. The active period 400 includes an upstream duration 410, an intermission duration 420, and a downstream duration 430. Each of the upstream duration 410 and the downstream duration 430 sequentially includes a RX interval 412, 432, a TX interval 414, 434, and an ACK interval 416, 436. However, in the wireless sensor network disclosed in the Korean Patent Application, there is a problem in that, in the case where linkage between consecutive two nodes is broken, it is difficult to perform link recovery.